Optimal immune responses require adequate ionic supply and carefully regulated ion-homeostasis. The adverse effects of low-Mg2+ conditions on immunity are well documented, but mechanistic insights into this Mg2+-sensitivity are lacking. The recently discovered protein TRPM7 is the unique fusion of an active Ser/Thr kinase with an ion channel, and a master regulator of Mg2+-homeostasis. TRPM7 has been shown to interact with several phospholipase C (PLC) isozymes. PLC proteins are at the heart of crucial signaling pathways required for the development and activation of virtually every immune cell type, including B-lymphocytes, which are the cellular architects of humoral immune responses. PLCg2 is central to B-cell receptor (BCR) signaling, and mediates B- cell maturation as well as activation. We propose that TRPM7-kinase modulates BCR- signaling in accordance to the availability of Mg2+ through Ser/Thr phosphorylation of PLCg2. The regulation of PLCg2 by Tyr-phosphorylation has been amply characterized, but its modulation by Ser/Thr phosphorylation is only postulated, although highly probable, since the vast majority of cellular phosphorylation events involve Ser/Thr residues. We have gathered preliminary experimental evidence in cell lines supporting our main hypothesis that the C2-domain of PLCg2 is a substrate of TRPM7-kinase, resulting in the Mg2+-sensitive modulation of BCR-elicited Ca2+-responses. This proposal aims at further exploring the effect of this novel phosphorylation event on PLCg2's localization, Tyr-phosphorylation and enzymatic activity, as well as to investigate its physiological relevance in vivo using a complementation approach in an existing mouse model of PLCg2 deficiency. PUBLIC HEALTH RELEVANCE: Alterations in ion homeostasis have severe effects on human health, impairing the effectiveness and appropriateness of immune responses, and causing or exacerbating grave diseases such as cancer or diabetes. The planned studies will contribute to expanding our knowledge about molecular mechanisms allowing for the adjustment of immune responses to the availability of the essential and most abundant intracellular divalent cation Mg2+. Understanding these regulatory processes represent potential opportunities to develop novel immunomodulatory strategies for therapeutic intervention. Given its focus on B-lymphocytes, this proposal is relevant to conditions such as auto-immune diseases, or various forms of immunodeficiencies.